Well Test Analysis in Lean Gas Condensate Reservoirs: Theory and Practice

TLDR

Gas condensate reservoirs exhibit complex two‑fluid behavior when produced below the dew point, with distinct mobility zones and challenging well‑bore phase redistribution that obscures reservoir, boundary, and fluid effects in well‑test data. The study compares theoretical well‑test responses from compositional simulation with observed data from over twenty gas‑condensate reservoirs to evaluate vertical and horizontal well behavior. An interpretation framework combining time‑lapse analysis, deconvolution, two‑ and three‑region composite models, a Voronoi‑grid numerical simulator, multilayered analytical models, and compositional simulation identifies the causes of pressure behavior. Condensate drop‑out reduces productivity non‑reversibly, partially offset by capillarity effects, and the methodology enables quantification of these effects and extraction of reservoir‑simulation parameters such as gas relative permeabilities, critical oil saturation, and base capillary number.

Abstract

Abstract Gas condensate reservoirs exhibit a complex behavior when wells are produced below the dew point, due to the existence of a two-fluid system, reservoir gas and liquid condensate. Different mobility zones develop around the wellbore corresponding respectively to the original gas in place (away from the well), the condensate drop-out, and capillarity number effects (close to the well). Condensate drop-out causes a non-reversible reduction in well productivity, which is compensated in part by capillarity number effects. All these effects can be identified and quantified from well test data. Tests in condensate reservoirs, however, tend to be difficult to interpret. Build-up and/or drawdown data are usually dominated by wellbore phase redistribution effects and the main analysis challenge is to distinguish between reservoir effects, boundary effects, fluid behavior and wellbore phase redistribution perturbations. The paper compares theoretical well test behaviors in vertical and horizontal wells as obtained from compositional simulation with actual behaviors selected from more than twenty different gas condensate reservoirs. An interpretation methodology is described, which uses time-lapse analyses, deconvolution and different analytical and numerical tools to identify the probable causes of the pressure data behavior: two-region and three-region analytical composite models to represent the various mobility zones around the wellbore; a voronoi-grid numerical simulator to represent discontinuous boundaries; a multilayered analytical simulator to account for the geological description and a compositional simulator to verify the fluid behavior. It is shown that, in addition to the usual well test analysis results, it is possible to obtain parameters required for reservoir simulation and well productivity forecasting, such as gas relative permeabilities at the end point, critical oil saturation, and the base capillary number.